The invention relates to a process for electrolessly metallizing sheetlike textile substrates, in particular, non-woven or needle felt webs, in which the substrates are activated and subsequently electrolessly metallized in a reductant-containing metallizing solution.
The surfaces of the fibers of textile substrates, in particular, those made of plastics fibers, are known to be non-electroconductive, and therefore cannot of themselves initiate a chemical metal deposition. To prepare the electroless (chemical) metallization, the fiber surfaces are therefore activated in a first step by nucleation with a catalytically active substance. Activation is possible not only with ionic and/or colloidal, but also with organic adducts of the elements of subgroups I and VIII of the periodic table of the elements. The elements generally used are gold, silver, palladium, platinum and copper. A particularly preferred activating metal is palladium in the form of a sol, in the form of an organometallic compound, or in particular, in the form of aqueous solutions which contain salts of palladium and tin.
After the substrate has been impregnated with the activating solution, the excess activating solution is removed, and the substrate is, if desired, treated with an accelerating solution and possibly rinsed, and thereafter dipped into a customary metallizing bath, such as those based on copper, silver, and in particular, nickel being preferred.
The preparation and the composition of activating solutions are familiar to those skilled in the art and described, for example, in German Published Application DE-AS No. 1,197,720, or German Laid-Open Application DE-OS No. 2,743,768. Similarly, a wide variety of metallizing solutions are known to those skilled in the art. Aside from complexing agents and agents for setting the pH, they contain mainly a dissolved salt of the metal to be deposited and a reductant. The reductants used are customarily sodium hypophosphite or sodium borohydride, but can also include alkylaminoboranes or formalin.
The chemical deposition of metal starts in those areas where the metallizing solution comes into contact with the catalytically active nuclei present on the fiber surface. However, hydrogen evolution generally also takes place in competition with the chemical deposition of metal. It is thus necessary not only to ensure an adequate supply of the metal ions to be reduced, together with the reductant, to the fiber surface, but also to guarantee the removal of the gaseous hydrogen formed in the course of the competing reactions by the catalyst particles adhering to the fiber.
It is true that the chemical deposition of metal on individual fibers is perfectly unproblematic. However, difficulties result in general when the totality of the fibers which make up a textile substrate, in particular a non-woven or needle felt, are to be metallized. The porosity of non-wovens or needle felts is customarily between 40 and 97%. If the fibers are very thin, for example, 1 to 4 dtex, and the fiber surfaces to be metallized become correspondingly large, the transport of the hydrogen bubbles out of the interior of the textile substrate can be impaired or slowed down. As a consequence, the accumulated hydrogen bubbles tend to block the access of further metals ions and ions of the reductant to the fiber surface. In these areas, metallization is then insufficient.
In known methods, the removal of the resulting hydrogen is facilitated in a chemical metallization of plastics fiber surfaces of a non-woven or of a needle felt by winding a previously activated non-woven or needle felt web of a certain length and width in spiral form onto a runner, in such a way as to include a layer of porous corrugated separator between pairs of layers of the non-woven or needle felt web. The runner thus produced is provided with an outer collar for shape stabilization, and is subsequently dipped perpendicularly into the metallizing solution. During the process of metallization, the hydrogen, which evolves at a very lively rate, can exit out of the interior of the substrate into the channels of the corrugated separator and rise upwards therein and escape from the metallization vessel. Nonetheless, the hydrogen formed does not escape quickly enough through the channels of the corrugated separator or does not leave the interior of the non-woven or needle felt to a complete enough extent.
Further, the addition of additives, such as wetting agents, to the metallizing solution or variation of the rate of metallization through temperature change in the solution did not bring about complete elimination of excessively non-uniform hydrogen evolution and/or removal. As a direct consequence thereof, the chemical metallizations in the individual zones in the interior of the textile substrates, in particular on non-wovens or needle felts, are excessively non-uniform as well, such that the fiber surfaces in individual zones in the interior of the textile substrate are not covered with a continuous metal coat. In these areas, the fiber surfaces then do not have the desired metallic properties such as, for example, thermoconductivity, electroconductivity, magnetic action, screening functions, electroplatability and the like.
An object of the present invention is to provide a process for the electroless metallization of sheetlike textile substrate, in particular non-wovens or needle felts, in which an adequate coating with a chemically deposited metal is obtained on the totality of the fiber surfaces of the textile substrate without individual zones or fibers of the non-woven or needle felt being metallized incompletely or non-uniformly or non-continuously.
This object is achieved by providing a process including activating a sheetlike textile substrate, and then metallizing the activated sheetlike substrate by holding the substrate in a single-ply form or multi-ply form in a metallizing solution in a horizontal attitude or at an angle of up to 20.degree. with the horizontal.
As a result, the hydrogen can escape upwards along a very short path, namely, only the thickness of the single- or multi-ply textile substrate, and the accumulation of relatively large gas cushions in the interior of the substrate, i.e., the non-woven or needle felt web, is avoided.
If a plurality of superposed substrates are metallized simultaneously, it is further advantageously contemplated in certain preferred embodiments to facilitate gas removal by holding the substrates in the solution spaced from each other. In certain preferred embodiments, it is contemplated to provide this spacing by the interposition of corrugated perforated separators or of wire networks between the substrates.
The angle by which the substrates deviate from the horizontal should not be greater than 20.degree.. If the substrates are at too steep an angle in the solution, it is possible for gas cushions to accumulate in relatively large areas of the textile material.
It is contemplated to improve the uniformity of metallization in a conventional manner by agitating the metallizing solution, for example by means of circulation pumps, or by periodically rocking or tilting the entire metallizing vessel. Since the textile substrates, in particular, non-woven or needle felt webs, can float to the surface of the metallizing solution as a result of gas evolution, it is contemplated to hold the substrates in the solution. This is achieved most simply by holding down the substrates beneath the surface of the liquid by a grid which can be locked in place in the metallizing vessel or which, on account of its weight, exerts a downward pressure on the substrate. A further possibility contemplated includes fixing the textile substrate in a rigid frame and holding it in the solution by means of this frame.
After the metallization has ended, the substrates are removed from the solution and converted into the end products in a conventional manner, for example, by washing, drying and confectioning.
The process is suitable for metallizing all fiber materials which can also be metallized using the existing processes. Examples of these fibers include non-wovens or needle felts made of polyethylene, polypropylene, polyamide, polyacrylonitrile, nylon, aramid and the like. The process is particularly effective in the case of non-wovens or needle felts which have a porosity between 40 and 97%. It is in the metallization of such materials that the process offers the greatest advantages.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.